There are several pieces of equipment used for resuscitating purposes which are well known in the medical art. The most commonly used is the AMBU, which consists of a hand piece coupled to a rubber bag, an inlet port for admitting oxygen from a remote source, and a face piece to admit oxygen to the patient. In an AMBU, the oxygen is admitted under pressure to the patient by compressing the rubber bag. While some resuscitating devices are provided with a pressure monitor so that the oxygen pressure admitted to the patient can be monitored, others are provided with a pressure valve so that the oxygen pressure admitted to the patient can be controlled. In most of the AMBUs there is neither a pressure monitor, nor a pressure control valve, so that the oxygen pressure must be "controlled" on the basis of the experience of the physician or medic utilizing the device. It is known, however, that the higher the oxygen pressure, the higher will be the peak inspiratory pressure imposed to the patient, so that a means of controlling the oxygen pressure is highly desirable.
When utilizing these prior art devices, the fraction of oxygen inhaled (FiO.sub.2) is not constant, since in most cases the oxygen admitted will be mixed the surrounding, ambient air. Another disadvantage of the prior art devices is that such equipment does not permit control of positive end-expiratory pressure in cases of patients under endotracheal intubation, which can be a serious disadvantage in some cases such as newborn infants, and particularly premature infants.